International patent application WO 96/033010 discloses a disc brake that is preferably operable by means of an electric motor by way of a reducing gear system. The special features of the prior art brake involve that the rotor of the electric motor has an annular design and embraces the reducing gear system in a radial way. These measures allow considerably shortening the axial overall length of the actuating unit. The above-mentioned publication, however, does not provide any indication about how defined clamping forces can be applied in the operation of the state of the art brake.
The publication ‘Electromechanical Brake System: Actuator Control Development Systems’, SAE Technical Paper Series 970814, discloses a control system for the application of defined clamping forces in an electrically operable brake which is provided by a cascaded arrangement of several controllers. A clamping force sensor is provided to determine the actual value of the clamping force applied by the actuator, while a position sensor is used to determine the actuator position or the actuator speed, respectively. The offset value of the clamping force sensor is determined with the brake opened and then subtracted from the clamping force sensor value during the normal operation. In the prior art control system, the distance between the first friction surface (friction lining) and the second friction surface (brake disc) in the initial position of the system, meaning the so-called air gap, is determined indirectly in such a way that the position is chosen as a contact position and, respectively, as the position sensor offset, in which position the clamping force sensor value that is cleared of the offset falls under a predetermined threshold value.
The time-consuming initialization of the sensors (clamping force sensor, position sensor) used to determine the clamping force and the actuator position is considered as a shortcoming in the prior art control system. Further, the control concept does not take the non-linear characteristic curve of rigidity of the system into account.